1. Field of the Invention
This invention relates to a communication control device which is useful in an information transmission system and which can automatically decide its self-address, and a system for automatically deciding a self-address by using the device. Hereinafter; a communication control device is sometimes referred to as merely "a control device".
2. Description of the Prior Art
FIG. 1 illustrates a so-called home bus system which is a typical example of an information transmission system. The home bus system of FIG. 1 comprises: a home bus 1 which consists of information transmission lines for transmitting information messages; control devices 2 and 3 which communicate with each other through the home bus 1; and terminal devices 4 and 5 such as CRTs or sensors which are respectively connected to the control devices 2 and 3.
As shown in FIG. 2, the control devices 2 and 3 comprise a home bus transmit/receive unit 13, a terminal connecting unit 14, a processing unit 15, and an address setting unit 16. The transmit/receive unit 13 transmits and receives control signals on the home bus 1 in accordance with the prefixed signal format. The terminal connecting unit 14 is connected with the terminal device 4 or 5 to send and receive control signals to and from the terminal device 4 or 5. The processing unit 15 is connected to the units 13, 14 and 16 to control the transmission process. The address setting unit 16 sets a self-address which is an inherent and unique piece of data allocated to the control device 2 or 3, depending upon the kind of terminal device 4 or 5. When the terminal device 4 or 5 is a meter-reading terminal which reads an indicating value of a meter such as a wattmeter, the self-address must be selected from four values, 1C.sub.H, 9C.sub.H, 5C.sub.H and DC.sub.H.
In the system of FIG. 1, both the terminal devices 4 and 5 are meter-reading terminals, and the self-address of the control device 2 connected to the terminal device 4 and the control device 3 connected to the terminal device 5 are previously set to 1C.sub.H and 9C.sub.H, respectively. When a further control device connected with another terminal device which is a meter-reading terminal is additionally connected to the home bus 1 of the system in which self addresses have been set as mentioned above, the self-address of the further control device is manually selected to be 5C.sub.H or DC.sub.H, so as not to overlap with the self-addresses 1C.sub.H and 9C.sub.H which have been already set in the system.
In a case where such a conventional control device is to be connected to a bus in an information transmission system, therefore, the operator must know self-addresses of control devices which have been already connected to the bus. Thereafter, he selects a value for the self-address of the newly connected control device so that it does not overlap with the self-addresses of the previously connected control devices, and then he sets the selected value as the self-address by operating DIP switches disposed in the newly connected control device. As described above, a conventional control device has disadvantages that the procedure of connecting a conventional control device to a bus is complicated and that there is a danger of erroneously setting a self-address.